Endocardial pacing leads may be classified in two broad categories: permanent pacing leads and temporary pacing leads. Permanent and temporary pacing leads are generally characterized in having different physical structures, materials and configurations. Structural differences between the two general types of pacing leads are driven primarily by cost considerations and the different natures of the applications for which the two types of leads are employed. In 1996 U.S. dollars many temporary pacing leads have actual or suggested retail prices in the U.S. that vary between about $75 and $200. In contrast, many permanent pacing leads sold in the U.S. have actual or suggested retail prices exceeding $1,000. Most temporary pacing leads are used for one week or less and then disposed of, while permanent pacing leads often remain implanted and functioning in patients for five years or longer.
When a permanent pacing lead is implanted in a patient, a pacemaker and an electrical connection between the pacing lead and the pacemaker are generally embedded within the body. Permanent pacing leads are commonly implanted with the aid of stylets that increase the speed and accuracy of lead electrode placement. Moreover, once the lead has been implanted and the stylet withdrawn, the remaining lead body becomes flexible and does not retain the stiffness imparted by the stylet. Thus, stylets are highly desirable and often used in permanent leads.
When implanting a permanent pacing lead, a peripheral vein such as the left or right subclavian vein is punctured by an introducer through an incised portion of the skin. A prior art "catheter" or a lead containing a stylet is inserted through the introducer. When a prior art "catheter" is used, the "catheter's" distal end is held at the apex of the right ventricle or right atrium while a temporary lead is inserted through the prior art "catheter" until the distal end of the lead engages and is lodged or otherwise affixed to the endocardium of the right ventricle or right atrium; the prior art "catheter" is then withdrawn. If a lead having a stylet is used, the distal end of the lead is guided to the apex of the right ventricle or the atrial appendage in the atrium, the lead electrode tip is affixed to the endocardium and the stylet is removed. We use the term "catheter" above respecting the prior art because prior art catheters were generally tubes formed from rubber, neoprene or plastic tubes that had no inner braiding or other structural strengthening means that permitted the control or transfer of torque. Thus, the term "catheter," as it applies to the prior art discussed here, does not include within its scope "guide catheters" of the present invention that, contrariwise, do permit the transfer and control of torque from their proximal to distal ends.
It is notable that no permanent leads known to the inventors of the present invention have employed catheters for several decades. The are several reasons why such prior art catheters are no longer used. Prior art catheters could not provide as much torque control or transmission as stylets. The thick sidewalls and correspondingly large diameter of prior art catheters rendered such catheters large in diameter. In fact, many such catheters had such large diameters that they could not be inserted in the cephalic veins of a significant number of patients.
Temporary transvenous endocardial pacing leads are generally used prior to pacemaker implant surgery or in emergency treatment of heart arrythmias and myocardial infarction. In temporary pacing, the distal end of a temporary pacing lead is inserted transvenously in the body using some of the techniques described above for permanent leads while the proximal end is located outside the body where electrical and mechanical connections to an external temporary pacemaker are made. The positive and negative connectors at the proximal end of the temporary lead are connected to the terminals of the temporary pacemaker or patient cable provided for strain relief or extension purposes. (A patient cable is usually, in turn, then connected to a temporary pacemaker.) The temporary pacemaker provides pulses of electrical energy to stimulate the endocardium through the temporary pacing lead. The stimulation rate, output amplitude and sensitivity of the temporary pacemaker are then adjusted. Typically, a temporary pacing lead is extricated and withdrawn from a patient when a permanent, implantable pacemaker and corresponding permanent lead are implanted, or when the need for pacing no longer exists.
Epicardial pacing leads are often used in temporary pacing applications following transthoracic surgery, where the electrode is affixed to the surface of the heart. It is an advantage of endocardial leads that they typically require lower stimulation thresholds to pace the heart than those required with epicardial leads because endocardial leads provide lower stimulation thresholds overtime. Temporary pacing leads should not be reused, are designed to be disposed of after a single use, and are not designed for use over prolonged periods of time.
While low cost temporary pacing leads have been widely used for decades, low cost temporary leads known heretofore have never been used in conjunction with stylets or guide catheters. This may be because stylets and guide catheters add considerable, excessive and therefore unaffordable cost to known temporary pacing lead products, or simply because heretofore no one has previously conceived of combining temporary pacing leads and guide catheters or stylets.
Prior art low cost temporary pacing leads have never included active fixation structures at their distal ends for attaching the lead to the endocardium, despite the clear advantages and benefits of such devices.
Among other reasons, cost considerations have prevented the use of active fixation devices in temporary pacing leads. Active fixation devices generally require expensive sheathing or shrouding structures to protect venous and cardiac tissue from the device during implantation. Such sheathing structures include those retracted to expose the device when the distal end of the lead is positioned at the affixation site, as well as glycol-containing compounds disposed about the active fixation device that slowly dissolve upon being immersed in a warm, sanguine medium. Examples of mechanical sheathing structures for transvenous pacing leads include various distal end sheaths and helical electrodes retractable by stylet means.
Temporary pacing leads known in the prior art fall into two broad categories: (1) coaxial temporary pacing leads, and (2) temporary pacing leads having one, two or three lumens.
Coaxial temporary pacing leads are characterized in having inner and outer conductors separated by an electrically insulative material, where the inner conductor typically comprises three twisted or stranded wires, and the outer conductor typically comprises a woven metallic mesh formed of 16 woven wires. The outermost layer is electrically insulative and is typically formed of urethane, polyurethane or polyethylene. Coaxial temporary leads are usually 6 to 7 French in diameter, but may be as small as 4 French in diameter. Examples of coaxial temporary pacing leads include MEDTRONIC.RTM. Model Numbers 6704, 6704A, 6705 and 6705A TEMPTRON.RTM. leads described in MEDTRONIC publication reference number MC 78-PE-0086c 179562-001, the disclosure of which is hereby incorporated by reference in its entirety. FIG. 1 shows a side cutaway view of a prior art coaxial temporary lead. FIG. 2(a) shows a cross-sectional view of the lead of FIG. 1.
Single, double or tri-lumen temporary pacing leads are characterized in having one, two or three lumens for housing two or more electrical conductors. The conductors are usually electrically insulated from one another by their respective layers of electrical insulation or by lead body electrical insulation. Each conductor typically comprises up to 8 twisted or braided wires disposed within the insulation. Examples of single and double lumen temporary pacing leads include DAIG.TM. temporary pacing lead model numbers 401674, 401675, 401665 and 410666, USCI.TM. temporary pacing lead model numbers 7153, 7151, 7157, 8154, 7150, 8153, 6221, 7406 and 6222, TELECTRONICS CORDIS.TM. temporary pacing lead model numbers 370-230, 370-132, 370-330, 370-136 and 370-420, and ELECATH.TM. temporary pacing lead model numbers 11-KSS5, 11-KSS6, 11-KSS4, 22-0865 and 22-0866. PROCATH CORPORATION.TM. of Berlin, N.J. also manufactures single lumen temporary pacing leads. FIG. 2(b) shows the cross-sectional structure of a prior art single lumen temporary lead having two electrical conductors disposed side by side. FIG. 2(c) shows the cross-sectional structure of a prior art tri-lumen temporary lead having three separate conductors.
Some ideal attributes of a temporary pacing lead include: (1) small lead diameter; (2) secure placement of the tip electrode in the selected heart chamber; (3) high degree of steerability, control and torque transfer during implantation; (4) minimal damage to vein, heart valve and endocardial tissue during implantation; (5) reliable conduction of electrical impulses during use; (6) easy removal from the heart chamber with minimum tissue damage, and (7) low cost.
Small diameter leads are desirable for several reasons. A vein has a finite diameter and thus a finite cross-sectional area for receiving one or more leads. A small diameter temporary lead is accommodated more readily in, and impedes less the flow of blood through, a vein than does a large diameter temporary lead. A small diameter lead also provides minimum interference with the flow of blood through a venous vessel or a heart valve. Large diameter leads are known to adversely affect heart valve operation. Finally, some patients already have at least one implanted lead when an additional temporary pacing lead must be implanted; having a small diameter lead becomes a significant advantage under such circumstances.
Large diameter temporary leads are more likely to rub against and dislodge permanently implanted leads during lead removal than are small diameter temporary leads. Large diameter temporary leads present a greater mass of foreign material to the body, and thus present a higher risk of occlusive thrombosis, scar tissue formation and thrombotic pulmonary embolism than do small diameter temporary leads. Because large diameter leads are generally stiff, they more likely to perforate veins or cardiac tissue, are more prone to lead fatigue and subsequent failure, and may take a long time to place. A primary cause of lead failure is the crushing of large diameter lead bodies between the relatively small space between the clavical and first rib.
Despite the numerous advantages of small diameter leads described above, in practice small diameter leads have proved difficult to manufacture, and are frequently unreliable and difficult to place. For example, many prior art small diameter leads are too pliable to permit sufficient steerability and control for accurate lead placement. Additionally, the high degree of flexibility and limpness characterizing many small diameter leads may lead to excessive time and effort being required for lead placement. In consequence, the cost for the procedure rises and the patient is progressively exposed to more risk factors as the amount of time expended to complete the procedure increases. Finally, most small diameter leads do not transfer sufficient torque between the proximal and distal ends to permit the tip electrode to be affixed to cardiac tissue at a selected site with any degree of accuracy.
Secure placement of the tip electrode in the selected heart chamber is required to assure appropriate and reliable depolarization or "capture" of cardiac tissue by electrical stimuli delivered by the temporary pacemaker. Known temporary transvenous leads suffer from a relatively high rate of dislodgment from sites adjacent or on the endocardium. This is not surprising in view of the fact that no prior art temporary transvenous pacing leads utilize active fixation devices to positively secure the electrode tip to the endocardium. Instead, known temporary pacing leads rely on force provided by a bent or curved lead body as a means of pushing the distal electrode tip against endocardial tissue. If the pacing lead body or tip shifts position as a result, for example, of patient postural changes, the tip electrode may disengage or float away from the endocardium. This, in turn, may result in a loss of capture, or in a reduction of the degree of electrical coupling between the electrode and endocardium.
Treadmill tests can yield valuable information concerning a patient's health and diagnosis that may not be obtained in any other way. Despite their clear benefits, however, physicians rarely prescribe treadmill tests for patients having implanted temporary transvenous pacing leads because temporary pacing leads typically become dislodged easily when patients are ambulatory or otherwise move about. Furthermore, it is common that patients having implanted temporary pacing leads cannot be paced in the DDD mode because at least one of the leads dislodges or becomes poorly coupled electrically to the endocardium after the lead implantation procedure has been completed. This is because it is very difficult to maintain pacing in the atrium over any appreciable length of time when the pacing lead has no means for fixation to the atrial wall.
It is desirable that temporary pacing leads have a high degree of steerability, control and torque transfer to permit relatively quick and accurate placement of the electrode tip at the desired site within the heart, and the initiation of temporary pacing with minimum delay and tissue trauma. Speed and accuracy of lead placement become especially important when attempting to restore a patient's heartbeat under emergency conditions. In the past, there have been a limited number of sites in the atrium and ventricle where lead placement could be effected. The accuracy of where the pacing lead is placed in the atrium or ventricle thus assumes considerable importance.
There are two known means of achieving a high degree of permanent lead steerability and control: (1) placing a stylet in a lumen inside the lead body, and (2) using a guide catheter in conjunction with the lead. Stylets and guide catheters, however, impart significant additional and unaffordable cost to temporary pacing leads, and as a result are not employed in known temporary pacing leads.
Physicians often rely on the tactile feedback and "feel" provided by the lead during implantation for accurate and quick placement. Because of their limpness and excessive flexibility, known small diameter pacing leads typically provide virtually no tactile feedback or "feel" to physicians. Small diameter pacing leads are also notoriously poor at transferring torque between their proximal and distal ends. As a result of the foregoing factors, known small diameter pacing leads generally prove difficult to accurately and quickly place.
Contrariwise, and owing to their stiffness, known large diameter pacing leads often provide a high degree of tactile feedback and "feel." Unfortunately, large diameter pacing leads typically lack sensitivity in the feedback and "feel" they provide. Thus, while known large diameter pacing leads often provide good torque transfer and a high degree of tactile feedback, typically they are also incapable of providing the sensitivity required for a physician to discriminate between endocardial tissue and venous tissue. Consequently, the risk of inadvertently perforating venous or cardiac tissue is made greater with known large diameter pacing leads. Additionally, and owing to their stiffness, large diameter leads exert forces on tip electrodes which promote the growth of scar tissue which, in turn, increases pacing thresholds.
Ideally, temporary pacing leads should cause no damage to vein, heart valve and cardiac tissue during implantation. The temporary lead should have a highly flexible and soft distal tip that readily follows the direction of venous blood flow. Such directional following is often referred to as "floating" the lead or catheter through the venous system. A soft flexible distal tip on the lead or catheter may help prevent trauma to the surrounding venous and cardiac tissues as the lead is directed to the fixation site.
Temporary pacing leads should reliably conduct electrical pulses from the pacemaker even when sutures at the lead anchor suture site are drawn too tight, the lead is stressed by excessive patient movement, or when the pacemaker or attached lead is subjected to rough handling by hospital personnel. Temporary pacing leads are designed for a single use over a limited duration of time, and therefore are typically not constructed of materials that are as biostable, durable, strong or robust as those used in permanent pacing leads. Thus, known temporary pacing leads tend to fail more frequently than permanent pacing leads. Many failures of known temporary pacing leads are caused by fatigue and breaking of electrical conductor wires, electrical insulation that cracks or splits, or electrodes that become pitted or corroded.
When the need for temporary pacing no longer exists, the distal end of the temporary pacing lead should ideally be easily removable from heart chamber. Some known temporary pacing leads suffer from the disadvantage of occasionally damaging heart tissue upon being extracted from the tribiculae in which they are lodged. Other known temporary pacing leads having curved or J-shaped ends for pushing the tip electrode against the endocardium occasionally prove difficult to remove from the heart without at least some tissue trauma occurring.
Finally, temporary pacing leads should be available at low cost, especially since they are used only one time, and then for a very limited duration of time. Heretofore, providing a temporary pacing lead at low cost that has all or most of the foregoing desired attributes has proved impossible, despite the obvious and clear motivations for doing so.
Not surprisingly, no temporary pacing lead known in the prior art has most or all of the above-enumerated desirable safety, performance and cost attributes. What is needed is a low cost, yet still safe, reliable, small diameter, highly steerable, easily removable temporary transvenous endocardial pacing lead capable of reliably capturing, pacing and sensing the heart while causing minimum trauma to heart tissue during implantation and removal.
Transvenous endocardial leads and fetal scalp electrodes are well known in the art, some examples of which may be found in the issued U.S. Patents listed in Table 1 below.
TABLE 1 ______________________________________ Prior Art Patents U.S. Pat. No. Inventor(s) Issue Date ______________________________________ 3,348,548 Chardack Oct. 24, 1967 3,737,579 Bolduc June 5, 1973 3,769,984 Muench Nov. 6, 1973 3,815,611 Denniston, III June 11, 1974 3,827,428 Hon et al. Aug. 6, 1974 3,893,461 Preston July 8, 1975 3,903,896 Harmjanz Sept. 9, 1975 3,915,174 Preston Oct. 28, 1975 4,010,755 Preston March 8, 1977 4,106,512 Bisping Aug. 15, 1978 4,112,952 Thomas et al. Sept. 12, 1978 4,180,080 Murphy Dec. 25, 1979 4,214,594 Little Jul. 29, 1980 4,233,992 Bisping Nov. 18, 1980 4,271,847 Stokes June 9, 1981 4,280,512 Karr et al. Jul. 28, 1981 4,282,885 Bisping Aug. 11, 1981 4,475,560 Tarjan et al. Oct. 9, 1984 4,602,645 Barrington et al. July 29, 1986 4,699,157 Shonk Oct. 13, 1987 4,762,136 Baker, Jr. Aug. 9, 1988 4,799,499 Bisping Jan. 24, 1989 4,819,661 Heil, Jr. et al. April 11, 1989 4,886,074 Bisping Dec. 12, 1989 5,099,839 Miyata et al. March 31, 1992 5,246,014 Williams et al. Sept. 21, 1993 5,261,417 Osypka Nov. 16, 1993 5,261,419 Osypka Nov. 16, 1993 5,314,462 Heil, Jr. et al. May 24, 1994 5,356,427 Miyata et al. Oct. 18, 1994 ______________________________________
All patents listed in Table 1 hereinabove are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.